[avahi] Add plugin for the Avahi mDNS/DNS-SD stack - #4427

Avahi is installed by default on Fedora, RHEL, Debian and Ubuntu, but sos has
no plugin for it and no other plugin references it. Neither its configuration
nor the services it advertises reach an sosreport, which matters when .local
name resolution or multicast service discovery is misbehaving.

Paths are taken from upstream. avahi-daemon/Makefile.am defines the
configuration file as $(pkgsysconfdir)/avahi-daemon.conf and the service
directory as $(pkgsysconfdir)/services, and static-hosts.c reads
AVAHI_CONFIG_DIR "/hosts".

Unit names come from the shipped units: avahi-daemon.service, which requires
avahi-daemon.socket, and avahi-dnsconfd.service.

avahi-browse --all --terminate --resolve records what the host can see on the
link at collection time, and avahi-daemon --check reports whether a daemon is
already running.

The daemon holds no credentials, so no forbidden paths or postproc handling are
needed.

Paths and unit names are taken from upstream rather than a running system.
avahi-browse --all --terminate --resolve produces network traffic at
collection time — if maintainers would prefer that behind --all-logs or a
plugin option rather than run by default, I am happy to move it.

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